Thomas Prolier

Thomas Prolier is a physicist at Argonne and the winner of a Department of Energy Early Career Research Award for his project, "Atomic Layer Deposition of Superconductors: an Innovative Approach to Improve the Performance of High Energy Accelerators."

How did you get into your particular branch of physics?Well, at school and later at university, I've always been good at math and physics, so I just followed that path. I studied superconductivity for my thesis, and I continued to study that for my first year at Argonne. Since then I've been also interested in chemistry, particularly how to grow compounds and superconductors in particular, by atomic layer deposition (ALD). It came to our minds that this could be used for accelerator physics, which is what the award was about.

Can you describe the work the award will be funding?We are applying this method, atomic layer deposition, to the improvement of high energy physics accelerator performance.

How would your proposal improve accelerators?Right now, large accelerators are too costly for all but a few labs to have them. Our proposal said, "We think we can make cavities cheaper and better." It's basically energy-saving and cost-saving for accelerators.

What exactly is atomic layer deposition?It's a manufacturing technique that allows you to lay down a film very precisely and evenly. This technique is the only one that can coat complex-shaped objects. Say you have a flat surface and spray it with a plume; the resulting layer will be fairly uniform. But if you spray an irregularly-shaped surface, with contours, the atoms will be unevenly distributed.

ALD, however, deposits the same amount everywhere. You can control the thickness and chemical composition very precisely.

How does it work?In a simple picture, say you want to apply a layer of a compound, which has multiple ingredients. You start with two chemical precursors, A and B. The A precursor is transported in its vapor state by a carrier gas—nitrogen or another inert gas. The precursor reacts with the surface of the sample but not with itself, and so you get one truly good coat of this A precursor on the entire surface of your sample.

Then you fill the chamber with the B precursor and it reacts with A, and once again not with itself. So you have a layer of A-B at the surface layer. For example, say you want to make a layer of aluminum oxide; you'd have a precursor for the aluminum as A and one for the oxygen as B. You can make one atomic layer of aluminum oxide at the surface, and you can continue to add layers, A-B, as long as you want. You could even have C and D precursors and alternate layers of A-B with C-D. The possibilities are almost endless.

What part of an accelerator would be made with ALD?Specifically, ALD could be applied to superconducting accelerating cavities. These are the chambers that accelerate the charged particles.

You have a series of cavities. Charged particles flow in, and each time they go through a cavity they get a boost. So more cavities, and therefore longer accelerators, increase the particles' speed. These cavities are made out of a superconductor, niobium, which costs about as much as silver. Each cavity costs maybe $100,000—so you can see an accelerator can get costly very fast.

Our proposal was to look for other compounds to replace niobium. This technique would coat the inside of these cavities with the stack of superconductor/insulators layers—only about 100 nanometers thick. Right now, our technique is to apply other compounds over a very thin layer of niobium. This means the cavities themselves cost more, but they perform better, so you can install perhaps half the number of cavities with the same total accelerating gradient.

The real improvement would be to replace the niobium with a better superconductor. There are a lot of them, but they're hard to grow and we are actively exploring alternative solutions.

We do have some preliminary results on cavities that show some improvement, so this is a starting point.

What's the importance of this to physics?If this works, it will be used all over the world: from large-scale accelerators that cost billions of dollars for each one to smaller-scale ones (maybe just a few cavities) used at universities. We could make accelerators better and cheaper, so more people can use them, and all the discoveries that go with them would be accelerated as well.

What's the big challenge for you?For me, it's that whatever I'm looking for will be used by the maximum number of people. That is, changing something in the day-to-day life of people. This could be solar cells, accelerators. The goal is to be useful; to leave something.

What are the best and worst parts of your job?The worst part is when the experiment doesn't work. The best is when it does work! [laughs]

I also like to try to explore different areas of science, hoping that I can bring something, and looking for the same motivation in others.

What do you do in your spare time?I take care of my child. I have one, so far, a son, just one year old. Otherwise, I do some painting with watercolors, oil and pastels. I also play the guitar; my favorite is flamenco. From time to time I have friends from Argonne who also play instruments and we get together to play, but that's very occasional.